The present invention generally relates to apparatus for zone refining polycrystalline semiconductor rods to produce monocrystalline semiconductor rods, and more particularly to improvements in said apparatus which permit the processing of larger diameter rods, and longer rods, without unduly increasing the overall height of the apparatus.
Conventional apparatus for zone refining includes an induction heating chamber. A rod holder for holding a polycrystalline semiconductor rod to be refined comes into the chamber from the top. A seed holder for holding a semiconductor seed crystal comes into the chamber from the bottom. The chamber has a door with a window, the window being necessary to permit the operator to watch the zone refining operation especially during the initial stage when the seed crystal is fused to the molten end of the semiconductor rod.
In a typical apparatus, a polycrystalline semiconductor rod on the order of 110 cm long can be refined. Such a rod is attached to the rod holder at its upper end, and a 6 mm diameter seed crystal is attached to the seed holder. A heavy RF induction heating coil of suitable design is positioned near the middle of the induction heating chamber. The chamber is purged and then either evacuated or filled with an inert gas, such as argon. The rod holder and rod are moved down so that the free end of the rod approaches the RF induction heating coil. The RF coil inductively heats and melts the bottom or free end of the semiconductor rod until a good molten droplet of semiconductor is formed. At that point, the seed crystal and seed holder moves up to the molten end of the rod within the heating zone of the RF coil. The seed crystal fuses and is pulled away to create a taper at the molten end of the semiconductor rod. Thereafter, the zone of the melt is moved up the rod by moving both the rod and the seed crystal downward. Relative movement between the seed holder and rod holder controls the diameter of the refined monocrystalline semiconductor rod and, in addition, the rod holder and seed holder can be independently rotated as both move downwardly with respect to the induction heating chamber.
In order to melt the semiconductor rod, the RF power required is substantial, and losses are minimized by providing the tank circuit just outside the induction heating chamber at the same level as the RF coil. This permits the power leads in the form of a coaxial cable between the tank circuit and the RF coil to be as short as possible.
The minimum height for conventional zone refining apparatus is at least for and often five times the length of the polycrystalline rod to be refined. The induction heating chamber itself must be twice as long as the rod to accommodate the full movememt of the rod through the RF coil. The fully extended positions of the rod holder and the seed holder are each equal to the length of the rod at a minimum.
Since the start-up time of a zone refining operation is very time consuming and requires monitoring by a highly skilled operator, a determined effort has been made to process larger and larger rods. Unfortunately, the overall height requirements for the zone refining apparatus poses a serious problem. In some cases, the size of the building housing the apparatus would have to be increased, an expense which may not be justifiable. Moreover, tall structures become laterally unstable, thereby posing serious problems of support and dimensional stability.
One possible solution is to move the RF induction heating coil rather than the semiconductor rod. If this solution were done then, in theory at least, the induction heating chamber could be made somewhat longer than the rod to be refined. The chamber would have to be longer than the rod to allow the starting position of the RF coil to be within view of the operator, to accommodate the rod holder and seed holder, and for any additional travel space required for the refining process. While this solution is at first appealing, it must be remembered that in order to melt larger diameter semiconductor rods, substantially high power at a very high frequency (from 2 to 4 megahertz) must be coupled into the rod by the induction heating coil. This means that if the RF coil is moved, a heavy flexible coaxial cable, or some other applicable means, must be used to connect the tank circuit to the RF coil. The expense and other problems, especially large power losses and possible arcing in the coaxial cable, make this approach to the problem very unattractive. Even under the best of circumstances, the cables needed to carry the power required to the RF coil will ordinarily result in a loss of 30 to 50 percent of the power; these numbers could vary depending upon the electrical circuits and machine design.
According to the present invention, the zone refining apparatus is provided with an induction heating chamber the longitudinal dimension of which is not determined by the length of the rod to be refined, but instead is limited only by considerations of space for the RF coil and related apparatus which must be mounted within the chamber, viewing space for the operator, and the heating effect on the structure above and below the chamber. More specifically, the semiconductor rod is moved and the RF coil is stationary, just as in the conventional apparatus. However, the semiconductor rod extends above the induction heating chamber initially and below the chamber at the conclusion of the refining process. An upper metal bellows extends from the top of the rod holder to the top of the chamber. A similar lower metal bellows extends from the bottom of the seed holder to the bottom of the chamber. The two bellows thus keep the working space both gas and vacuum tight. A telescoping group of steel cylinders are placed inside the lower bellows to protect the lower bellows from any molen semiconductor which may drop from the melt zone. The length of the telescoping cylinders collapsed is the same as the length of the bellows collapsed. Each bellows terminates in a flange that is bolted to the induction heating chamber. These bolts are easily undone for insertion and removal of the semiconductor rod.
In addition, the present invention incorporates the application of non-wearing magnetic fluid seals to be used with the top and bottom shafts, i.e., the semiconductor rod and the seed holder. Non-wearing magnetic fluid seals are used only as rotary seals, thus such seals are not suitable for conventional zone refiners where the shafts have linear as well as rotational displacements with respect to the seals. In refiners using bellows, the shaft seal assembly and the shaft move up and down with zero relative linear motion. The motion of the shaft is attained by collapsing and opening the bellows; thus the relative motion of the shaft with the seal is purely rotational.
Commonly used seals in conventional refiners are either O-ring type or carbon impregnated spring loaded teflon seals. In an O-ring type shaft seal system, too much of torque is exerted on the shaft which may result in a jerky operation, and may even produce chattering or vibration during the crystal growth process. For obtaining high vacuum with these seals, it is desired that a monolayer of vacuum grease be used with the O-rings. Any kind of grease used eventually ends up in the zone refiner chamber and at times becomes as a contaminant. Another disadvantage is when the small silicon chips cut through the O-ring surfaces making them less effective for good sealing. Spring loaded teflon have similar problems. Potential problems are encountered when the machine is switched from vacuum refining to gas refining. This is because of different design criteria of these seals under both applications.
On the other hand, magnetic seals are excellent for vacuum as well as for positive pressure conditions, and do not require any modifications in switching from one mode to another. They do not exert any torque on the shafts because there is no radial and peripherial forces involved. The sealing is done by the magnetic fluid which does not apply pressure. Since the fluid stays at the same place and does not ride up and down on the shaft, there is no contamination involved. Even in the case when small silicon chips get in the fluid its effectiveness is not reduced significantly.
To load the apparatus according to the invention, the upper bellows is extended fully to place the rod holder at its upper most position above the induction heating chamber. The upper bellows is then unbolted from the chamber and compressed from the bottom up thereby permitting the semiconductor rod to be fastened to the rod holder with the lower or free end of the semiconductor rod just projecting within the heating chamber. The upper bellows is then extended and re-attached to the top of the chamber. In a similar operation, the semiconductor seed crystal is attached to the seed holder. Once the lower bellows is attached to the bottom of the induction heating chamber, the lower bellows is compressed upward so that the seed crystal is in position to begin the zone refining process. During the zone refining process, the rod holder and seed holder move downwardly causing the upper bellows to compress and the lower bellows to expand. At the end of the refining process, the bottom bellows is unbolted from the bottom of the heating chamber and compressed downwardly permitting easy access to the refined monocrystalline semiconductor rod which is removed from the seed holder.